1. Field of the Invention
The present invention relates to an apparatus for forming a screw-thread(s) on or around a metal pipe (such as a steel pipe) and a method for forming the screw-thread(s) on or around the metal pipe using such an apparatus, and further relates to a fuel supplying pipe assembly being assembled with the metal pipe, on which is formed the screw-thread(s) being obtained by the described screw-thread forming apparatus and method therefor.
2. Description of Prior Art
Conventionally, a roll forming method and a punch forming method are known for forming a screw-thread(s) on a metal pipe.
In the roll forming method, as shown in FIG. 11(a), a metal pipe 101 is introduced to cover on a core metal 100, around which is formed a forming groove 100a, and as shown in FIG. 11(b), a protruded or convex forming portion 102a of a forming roller 102 is pushed on the metal pipe 101 when the metal pipe 101 begins rotation together with the core metal 100, so as to deform a threaded portion 103 following the forming groove 100a.
On the other hand, the present invention can be classified as a punch forming method, and one such method is already known as is disclosed in Japanese Laid-Open Patent No. Sho 61-253140 (1986).
Namely, as shown in FIG. 12(a), a plurality of forming punches 104 are provided, each having a forming edge portion 104a of curvature corresponding to that of the concave portion of the desired screw-thread, being freely movable in a direction toward one point (i.e., a central portion of a set) back and forth, and a metal pipe 101 is set or fixed at the center of the plural forming punches 104. Then, the plural forming punches 104 are shifted toward the central portion of the set to give pressure on the metal pipe 101 at the forming edge portion 104a of the punches, thereby forming the threaded portion by deforming the pipe 101 plastically.
In the roll forming method, pressure is applied to the metal pipe at the summit of the convex forming portion 102a of the forming roller so as to perform machining plastically with continuity. In other words, the summit of the convex forming portion 102a and the metal pipe 101 are in contact with each other under a condition of point contact or a form of contact similar thereto, as can be seen from FIG. 11(b), especially in a direction C thereof. Therefore, the material is extended (deformed) only in the portion where the summit of the convex forming portion 102a contacts with it, and the threaded portion is formed by the material being extended. As a result of this, the threaded portion is extremely reduced in the thickness thereof.
It is noted that the roll forming method is not appropriate in particular in a case when forming double threading, or wherein the threaded portions are not formed more than one turn around the outer periphery of the pipe (i.e. wherein threaded length-wise portions of the pipe 3 are formed, the threads not being physically continuous on the circumference of the pipe 3).
On the other hand, in the punch forming method, the curvature of the thread-forming portion of the forming punch is set to be equal to that of the convex curvature of the threaded portion to be formed, but it is smaller than that of the outer periphery on the metal pipe before the forming thereof. Therefore, as shown in FIG. 12(b), in initial steps of the machining process, the forming punches abut on the outer periphery of the metal pipe only at both ends p of each forming portion 104a thereof, and the forming is started from those portions. The material between the forming punches is not moved nor extended into such direction that it forms the threaded portion, however, as shown in FIG. 12(c), it rather extends or bulges outward in the radial direction.
And, as the material of the portion being expanded outward is finally protruding among or between the forming punches, it is therefore impossible to form the threaded portion uniformly.